1. Field of the Invention The present invention relates to a variable capacity refrigerant compressor adapted for being incorporated in an automobile climate control system to compress a refrigerant gas and, more particularly, relates to a construction of a cam plate of a variable capacity single-headed piston type refrigerant compressor in which the angle of inclination of the cam plate is adjustably changed with respect to a reference plane, i.e., a plane perpendicular to an axis of rotation of the drive shaft of the compressor, to vary the delivery capacity of the compressed refrigerant gas in compliance with a compression requirement of the climate control system.
2. Description of the Related Art
A typical conventional variable capacity refrigerant compressor is provided with a cylinder block for defining therein a plurality of cylinder bores, a front housing connected to the front end of the cylinder block to define therein a crank chamber for receiving a piston drive mechanism including a cam plate, and a rear housing connected to the rear end of the cylinder block to define therein a suction chamber and a discharge chamber. A drive shaft is rotatably supported by the front housing and the cylinder block to extend through the crank chamber. The drive shaft is arranged to be connectable to an external drive source such as an automobile engine. The cylinder bores of the cylinder block are provided for receiving a plurality of pistons to be reciprocated by the piston drive mechanism. The cam plate of the piston drive mechanism is mounted around the drive shaft within the crank chamber to be rotatable with the drive shaft, and can change its angle of inclination with respect to a plane perpendicular to the axis of rotation of the drive shaft. The cam plate is operatively connected to the pistons to reciprocate the pistons within the respective cylinder bores in response to the rotation of the drive shaft, so that the refrigerant gas is sucked, compressed, and discharged by the pistons.
The crank chamber is fluidly communicated with a suction pressure region and a discharge pressure region of the compressor, and accordingly, a part of the compressed refrigerant gas can be introduced into the crank chamber from the discharge pressure region, and a part of the refrigerant gas is delivered from the crank chamber into the suction pressure region. A capacity control valve is usually provided to regulate either an amount of introduction of the compressed refrigerant gas into the crank chamber or an amount of delivery of the refrigerant gas from the crank chamber into the suction pressure region. Thus, a pressure differential is adjustably produced between the crank chamber and the respective cylinder bores, so that the adjustable pressure differential acts on the back faces of the respective pistons. As a result, the angle of inclination of the cam plate is changed to cause a change in a capacity of the compressed gas discharged from the discharge chamber of the compressor toward the automobile climate control system.
The capacity control valve is arranged to perform the above-mentioned regulating motion in response to a detection of, e.g., a suction pressure of the refrigerant gas. Namely, the detected suction pressure is compared with a predetermined reference pressure, and either the introduction of the discharge pressure refrigerant gas into the crank chamber from the discharge pressure region or the delivery of the refrigerant gas from the crank chamber into the suction pressure region is adjusted to cancel a pressure difference between the detected suction pressure and the predetermined reference pressure.
Nevertheless, the described conventional variable capacity refrigerant compressor must often encounter a problem, to be solved, as set forth below with reference to FIG. 9 schematically illustrating a piston drive mechanism of a variable capacity refrigerant compressor.
As shown in FIG. 9, in the conventional refrigerant compressor, since the drive shaft is usually driven by the automobile engine, the speed of reciprocation of pistons 101 driven by a piston drive mechanism 100 is increased when the engine speed is increased. Thus, the pistons 101 exhibit an excessively increased inertial force F1 which acts on a cam plate 102 of the piston drive mechanism 100 to provide it with a moment M1 which increases an angle of inclination of the cam plate 102. Therefore, even if the capacity control valve of the compressor (not shown in FIG. 9) operates so as to maintain an intermediate capacity condition of the compressor, the cam plate 102 of the piston drive mechanism 100 is moved toward the position of maximum inclination angle thereof, due to the action of the moment M1, to increase the capacity of the compressor. As a result, the suction pressure decreases far below the predetermined reference pressure. Thus, the capacity control valve operates so as to quickly reduce a differential between the suction pressure and the predetermined reference by returning the cam plate 102 toward the position of the minimum inclination angle. However, the quick return of the cam plate 102 to the minimum inclination angle position causes an excessive reduction in the capacity of the compressor, and therefore, the suction pressure increases to a pressure level beyond the predetermined reference pressure. Thus, the capacity control valve operates so as to move the cam plate 102 toward the position of the maximum inclination angle, in order to reduce the suction pressure to a pressure level corresponding to the predetermined reference pressure.
It will be understood from the foregoing description that, when the speed of the automobile engine increases, the cam plate 102 carries out a hunting motion between two approximate positions close to the maximum and minimum inclination angle positions even if the capacity control valve operates so as to maintain an intermediate capacity condition of the compressor. Thus, a stable control of the capacity of the variable capacity refrigerant compressor cannot be achieved, and also, the hunting motion of the cam plate produces vibration of the various elements of the compressor and noise. Further, a large change in the capacity of the compressor causes a change in a torque exerted by the automobile engine, and accordingly, the driving performance of the automobile engine is affected.
At this stage, when the cam plate 102 is rotated by a drive shaft about its axis "L" of rotation, a centrifugal force "F2" acts on the cam plate 102 due to its own weight. The centrifugal force "F2" produces a moment "M2" which causes a reduction in the angle of inclination of the cam plate 102 with regard to a plane perpendicular to the axis "L" of rotation of the cam plate. Since the conventional cam plate 102 is generally made of a material of an iron system, the cam plate 102 is a heavy member. When the cam plate 102 is heavy, the centrifugal force "F2" acting on the cam plate 102 is necessarily large. Thus, the moment "M2" is large, and accordingly, during the high speed rotation of the automobile engine, the afore-mentioned moment "M1" is effectively canceled by the moment "M2". Thus, the afore-mentioned problem of the hunting motion of the cam plate 102 can be considered as being less than serious.
Nevertheless, all refrigerant compressors mounted on automobiles are required to reduce the weight in response to a requirement for reducing the total weight of automobiles. To this end, a proposal has been made to produce the cam plate 102 of a variable capacity refrigerant compressor by using a light material such as aluminum or an aluminum alloy having a small specific gravity. However, when the cam plate 102 is made of a light material a problem occurs in which, even when the automobile engine operatively connected to the compressor rotates at a high speed, the moment M2 produced by the light cam plate 102 is not large enough to effectively cancel the moment "M1". Thus, only the reduction in the weight of the cam plate 102 causes an effect on the driving performance of the automobile engine.